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Chapter 6 Ethernet Fundamentals

Chapter 6 Ethernet Fundamentals. By: Steven P. Luse. From its beginning in the 1970s, Ethernet has evolved to meet the increasing demand for high speed LANs. the same protocol that transported data at 3 Mbps in 1973 is carrying data at 10 Gbps.

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Chapter 6 Ethernet Fundamentals

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  1. Chapter 6Ethernet Fundamentals By: Steven P. Luse

  2. From its beginning in the 1970s, Ethernet has evolved to meet the increasing demand for high speed LANs. the same protocol that transported data at 3 Mbps in 1973 is carrying data at 10 Gbps. The success of Ethernet is due to the following factors: Simplicity and ease of maintenance Ability to incorporate new technologies Reliability Low cost of installation and upgrade 6.1.1 Introduction to Ethernet

  3. The original idea for Ethernet grew out of the problem of allowing two or more hosts to use the same medium and prevent the signals from interfering with each other. This problem of multiple user access to a shared medium was studied in the early 1970s at the University of Hawaii. A system called Alohanet was developed to allow various stations on the Hawaiian Islands structured access to the shared radio frequency band in the atmosphere. This work later formed the basis for the Ethernet access method known as CSMA/CD. 6.1.1 Introduction to Ethernet

  4. In 1985, the Institute of Electrical and Electronics Engineers (IEEE) standards committee for LANs published standards. They started with the number 802. Called Ethernet 802.3. This had to be compatible with the ISO/OSI model. To do this, the IEEE 802.3 standard had to address the needs of Layer 1 and the lower portion of Layer 2 of the OSI model. As a result, some small modifications to the original Ethernet standard were made in 802.3. The differences between the two standards were so minor that any Ethernet network interface card (NIC) can transmit and receive both Ethernet and 802.3 frames. Essentially, Ethernet and IEEE 802.3 are the same standards. 6.1.1 Introduction to Ethernet

  5. Ethernet is not one networking technology, but a family of networking technologies that includes Legacy, Fast Ethernet, and Gigabit Ethernet. Ethernet speeds can be 10, 100, 1000, or 10,000 Mbps. The basic frame format and the IEEE sublayers of OSI Layers 1 and 2 remain consistent across all forms of Ethernet. The abbreviated description consists of: A number indicating the number of Mbps transmitted. The word base, indicating that baseband signaling is used. One or more letters of the alphabet indicating the type of medium used (F= fiber optical cable, T = copper unshielded twisted pair). 6.1.2 IEEE Ethernet naming rules

  6. Ethernet operates in two areas of the OSI model, the lower half of the data link layer, known as the MAC sublayer and the physical layer. 6.1.3 Ethernet and the OSI model

  7. A collision domain is then a shared resource. Problems originating in one part of the collision domain will usually impact the entire collision domain. 6.1.3 Ethernet and the OSI model

  8. 6.1.3 Ethernet and the OSI model • maps a variety of Ethernet technologies to the lower half of OSI Layer 2 and all of Layer 1. Ethernet at Layer 1 involves interfacing with media, signals, bit streams that travel on the media, components that put signals on media, and various topologies. Ethernet Layer 1 performs a key role in the communication that takes place between devices, but each of its functions has limitations. Layer 2 addresses these limitations.

  9. 6.1.3 Ethernet and the OSI model • Layer 1 involves media, signals, bit streams that travel on media, components that put signals on media, and various topologies. Each of its functions has its limitations. Layer 2 addresses these limitations. • For each limitation in Layer 1, Layer 2 has a solution. • Layer 1 cannot communicate with the upper-level layers;Layer 2 does that with logical link control (LLC). • Layer 1 cannot name or identify computers;Layer 2 uses an addressing (or naming) process. • Layer 1 can only describe streams of bits;Layer 2 uses framing to organize or group the bits. • Layer 1 cannot choose which computer will transmit binary data, from a group in which all computers are trying to transmit at the same time;Layer 2 accomplishes this by using a system called Media Access Control (MAC).

  10. 6.1.3 Ethernet and the OSI model • Data link sublayers contribute significantly to technology compatibility and computer communication. The MAC sublayer is concerned with the physical components that will be used to communicate the information. The Logical Link Control (LLC) sublayer remains relatively independent of the physical equipment that will be used for the communication process.

  11. 6.1.4 Naming • Ethernet uses MAC addresses that are 48 bits in length and expressed as twelve hexadecimal digits. The first six hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor. This portion of the MAC address is known as the Organizational Unique Identifier (OUI). The remaining six hexadecimal digits represent the interface serial number, or another value administered by the specific equipment manufacturer. MAC addresses are sometimes referred to as burned-in addresses (BIA) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the NIC initializes.

  12. 6.1.5Layer 2 framing • Framing helps obtain essential information that could not, otherwise, be obtained with coded bit streams alone. • Which computers are communicating with one another • When communication between individual computers begins and when it terminates • Provides a method for detection of errors that occurred during the communication • Whose turn it is to "talk" in a computer "conversation"

  13. 6.1.5Layer 2 framing • The frame format diagram shows different groupings of bits (fields) that perform other functions. • The names of the fields are as follows: • Start frame field • Address field • Length / type field • Data field • Frame check sequence field 

  14. 6.1.5Layer 2 framing • All frames contain naming information, such as the name of the source node (MAC address) and the name of the destination node (MAC address). • In some technologies, a length field specifies the exact length of a frame in bytes. Some frames have a type field, which specifies the Layer 3 protocol making the sending request. • Data • The Frame Check Sequence (FCS) field contains a number that is calculated by the source node based on the data in the frame. This FCS is then added to the end of the frame that is being sent. • There are three primary ways to calculate the Frame Check Sequence number: • Cyclic Redundancy Check (CRC) – performs calculations on the data. • Two-dimensional parity– adds an 8th bit that makes an 8 bit sequence have an odd or even number of binary 1s. • Internet checksum – adds the values of all of the data bits to arrive at a sum.

  15. 6.1.6 Ethernet frame structure

  16. 6.1.7 Ethernet frame fields • fields permitted or required in an 802.3 Ethernet Frame are: • Preamble - is an alternating pattern of ones and zeroes used for timing synchronization in the asynchronous 10 Mbps and slower implementations of Ethernet. • Start Frame Delimiter - one-octet field that marks the end of the timing information, and contains the bit sequence 10101011. • Destination Address • Source Address • Length/Type • Data and Pad • FCS - contains a four byte CRC value that is created by the sending device and is recalculated by the receiving device to check for damaged frames. • Extension

  17. 6.2.1 Media Access Control (MAC) • There are two broad categories of Media Access Control, deterministic (taking turns) and non-deterministic (first come, first served). • deterministic protocols include Token Ring and FDDI. • Non-deterministic MAC protocols use a first-come, first-served approach. CSMA/CD is a simple system. The NIC listens for an absence of a signal on the media and starts transmitting.

  18. 6.2.1 Media Access Control (MAC) • The specific technologies for each are as follows: • Ethernet – logical bus topology (information flow is on a linear bus) and physical star or extended star (wired as a star) • Token Ring – logical ring topology (in other words, information flow is controlled in a ring) and a physical star topology (in other words, it is wired as a star) • FDDI – logical ring topology (information flow is controlled in a ring) and physical dual-ring topology (wired as a dual-ring)

  19. 6.2.2 MAC rules and collision detection/backoff • The access method CSMA/CD used in Ethernet performs three functions: • Transmitting and receiving data packets • Decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model • Detecting errors within data packets or on the network

  20. 6.2.3 Ethernet timing • The electrical signal takes time to travel down the cable (delay), and each subsequent repeater introduces a small amount of latency in forwarding the frame from one port to the next. Because of the delay and latency, it is possible for more than one station to begin transmitting at or near the same time. This results in a collision. • Full-duplex operation also changes the timing considerations and eliminates the concept of slot time. Full-duplex operation allows for larger network architecture designs since the timing restriction for collision detection is removed. • In half duplex, assuming that a collision does not occur, the sending station will transmit 64 bits of timing synchronization information that is known as the preamble. The sending station will then transmit the following information: • Destination and source MAC addressing information • Certain other header information • The actual data payload • Checksum (FCS) used to ensure that the message was not corrupted along the way • Stations receiving the frame recalculate the FCS to determine if the incoming message is valid and then pass valid messages to the next higher layer in the protocol stack.

  21. 6.2.4 Interframe spacing and backoff • The minimum spacing between two non-colliding frames is also called the interframe spacing. • The minimum spacing between two non-colliding frames is also called the interframe spacing.

  22. 6.2.5 Error handling

  23. 6.2.6 Types of collisions • Collisions typically take place when two or more Ethernet stations transmit simultaneously within a collision domain. A single collision is a collision that was detected while trying to transmit a frame, but on the next attempt the frame was transmitted successfully. • Three types of collisions are: • Local - collision on coax cable (10BASE2 and 10BASE5), the signal travels down the cable until it encounters a signal from the other station. • Remote - a frame that is less than the minimum length, has an invalid FCS checksum, but does not exhibit the local collision symptom of over-voltage or simultaneous RX/TX activity. This sort of collision usually results from collisions occurring on the far side of a repeated connection. • Late - Collisions occurring after the first 64 octets. The most significant difference between late collisions and collisions occurring before the first 64 octets is that the Ethernet NIC will retransmit a normally collided frame automatically, but will not automatically retransmit a frame that was collided late.

  24. 6.2.7 Ethernet errors • The following are the sources of Ethernet error: • Collision or runt – Simultaneous transmission occurring before slot time has elapsed • Late collision – Simultaneous transmission occurring after slot time has elapsed • Jabber, long frame and range errors – Excessively or illegally long transmission  • Short frame, collision fragment or runt – Illegally short transmission • FCS error – Corrupted transmission • Alignment error – Insufficient or excessive number of bits transmitted • Range error – Actual and reported number of octets in frame do not match • Ghost or jabber – Unusually long Preamble or Jam event

  25. 6.2.8 FCS and beyond • A received frame that has a bad Frame Check Sequence, also referred to as a checksum or CRC error, differs from the original transmission by at least one bit. In an FCS error frame the header information is probably correct, but the checksum calculated by the receiving station does not match the checksum appended to the end of the frame by the sending station. The frame is then discarded.

  26. 6.2.9 Ethernet auto-negotiation • 10BASE-T required each station to transmit a link pulse about every 16 milliseconds, whenever the station was not engaged in transmitting a message. Auto-Negotiation adopted this signal and renamed it a Normal Link Pulse (NLP). When a series of NLPs are sent in a group for the purpose of Auto-Negotiation, the group is called a Fast Link Pulse (FLP) burst. Each FLP burst is sent at the same timing interval as an NLP, and is intended to allow older 10BASE-T devices to operate normally in the event they should receive an FLP burst. • Auto-Negotiation is accomplished by transmitting a burst of 10BASE-T Link Pulses from each of the two link partners. The burst communicates the capabilities of the transmitting station to its link partner.

  27. 6.2.10 Link establishment and full and half duplex • Link partners are allowed to skip offering configurations of which they are capable. This allows the network administrator to force ports to a selected speed and duplex setting, without disabling Auto-Negotiation. 

  28. The End Good Luck on the Chapter Test

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